Generally, oil well pumping units run continuously or run in a predetermined cyclic fashion as set by a timer. In the cyclic mode, the pumping unit runs for some preset time, whether it was actually pumping fluid or not, and stays off for some other preset time. When the pumping unit is running and not pumping fluid, damage is incurred by the pump, rod string, gear boxes and other equipment and power is being consumed. Various control circuits have been proposed as shown in U.S. Pat. Nos. 2,947,931; 3,413,535; and 3,440,512, to shut off the power to the pump drive motor when the supply of liquid in the well has been pumped dry to prevent damage and the use of unnecessary power.
The present invention utilizes the fact that the average power required by the pump drive motor is less when no liquid is being produced from the well than when liquid is being produced, and provides a novel control circuit for measuring the average current and turning off the pump drive motor when the average current decreases below a preset level.
However, another problem is that oil well pumping units are usually found at locations and under conditions in which the electrical supply power to the pumping motor may vary considerably thereby adversely affecting the control circuit which is measuring the load power to the pump motor against some predetermined voltage level. The present invention provides a compensation circuit to offset the variations of the power supply voltage.